Mat intrusion detectors of the type to which the present invention finds application usually take the form of contact switches having spaced electrical contacts that connect when weight is applied to the mat and are biased apart to separate when weight is removed. An example of such an arrangement is shown in Miller U.S. Pat. No. 4,661,664, wherein two conductive sheets of aluminum foil are separated by a nonconductive intermediate spacial layer of insulation material which has openings therein through which the foil plates may be contacted when weight is applied to the mat. Other examples of similar arrangements are shown in U.S. Pat. Nos. 2,683,784; 2,783,327; 4,245,219; 4,401,896; 4,057,791 and 4,539,560.
Such mat switches detect intrusion by establishing contact between the plates. In order to set such alarms, it is necessary for the mat to be cleared of all objects, except perhaps during a brief timed period which is set by circuitry to give a delay sufficient to clear the area after arming. The switch is either "on" or "off", so leaving an object on the mat of sufficient weight to compress the plates will either prevent arming or leave the mat in an alarm state. When this happens, an intruder stepping onto the mat will not be detected because the status of the system will not change.
For spot protection of precious objects, such as museum treasures and the like, the operation of the mat can be reversed so that the normal, nonalarm state occurs when the plates are in contact and the alarm state occurs when the plates are separated. Thus, for example, an alarm will be given when a precious vase placed to assume a usual position of rest on a mat is raised up off the mat. Again, however, the mat is either in an alarm "on" or an alarm "off" status in accordance with whether the plates are contacting or not.
There is a problem with adjusting the sensitivity of such "on" or "off" contact mats because no discrimination is made in the electrical circuitry as to the level of the switch signal. And, while some attempt can be made to avoid alarm due to false triggers such as the passage of dogs or cats and the like, the capability for eliminating unwanted activation is limited. Changes can be made in the characteristics of the material separating the plates or making up the external mat covering to provide resistance against bringing the plates together until a certain threshold weight is applied. Such mechanical "tweaking" is, however, a coarse adjustment at most and does not provide for the on-site, fully adjustable, user controllable sensitivity desired in order to have an effective security system.
The utilization of capacitance effects in connection with intrusion detection is known for both vibration detectors and proximity detectors, both of which operate on principles different from the applied (or removed) weight principle employed in the contact mat, described above. A typical vibration detector will, for instance, have a first contact loosely positioned in the vicinity of a second contact so that momentary vibration of the first contact in response to intrusion-induced vibration will change the spacing and, thus, the capacitance of the contacts, to thereby trigger an alarm. Such a system is described in Gilcher U.S Pat. No. 3,846,780.
In Gilcher, a length of insulated electrical wire is loosely coaxially positioned for free movement within the interior of an electrically conductive tube which is buried underground. Mechanical disturbances of the wire within the tube due to ground vibration causes a measurable change in capacitance between the wire and the tube, which can be monitored to activate the alarm. Such vibration detectors are, however, sensitive to non-intrusion initiated vibrational displacements as well, such as those resulting from environmental changes, remote disturbances, or the unexpected acoustical noise resulting from the sudden turning on of an air-conditioner fan motor. Some provision can be made, as in Gilcher, to analyze the frequency of the detected vibration in order to eliminate some unwanted triggerings; yet, false alarms remain a problem and sensitivity is difficult to adjust.
Proximity detectors, on the other hand, use change in space capacitance as a detection mechanism. An electric flux is established through the air between a sensing wire or object (such as a metal safe or filing cabinet) to be protected and the surrounding building structure or ground area. When an intruder approaches, the flux is caused to flow through the higher dielectric constant of the intruder's body results in an increase in space capacitance that triggers the alarm.
Wisnia U.S. Pat. No. 3,496,381 shows an example of a proximity detector embodied as a capacitance-actuated door opener. In Wisnia, a signal plate in the form of a large flat metal sheet is mounted on the floor in front of a door opening and charged by an oscillator of preselected frequency to establish an electrical field with the surrounding area. When a person approaches the signal plate, the system responds to the increase in space capacitance by opening the door. Wisnia uses a second, generally identically-sized metal sheet between the signal plate and the floor, and maintained at the same potential as the first sheet, in order to mask the system against operation due to unwanted changes in capacitance due to traffic on the floor below. Nevertheless, because such devices require establishment of the electric flux in the surrounding air so that it can pass through an approaching body, they remain sensitive to false triggering due to normal changes in ambient conditions like temperature and humidity, and experience deterioration in performance when operated near large objects and close to walls. This is so, even though the sensitivity of some systems is intentionally set so that the intruder must actually touch the sensing wire or protected object to activate the alarm.